A technique by which a transistor is formed using a semiconductor thin film (also referred to as thin film transistor (TFT)) formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) and an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to the transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose semiconductor layer including an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) (an In—Ga—Zn—O-based amorphous oxide) is formed over a substrate is disclosed (see Patent Document 1).